The field of the present invention relates to data reading devices such as for example bar code scanning systems. The invention is applicable to both stationary or handheld scanners.
In a detection system such as a bar code scanning device employing a focusing lens, a light source such as a laser, laser diode, or non-coherent light source (e.g. light emitting diode) emits light which passes through and is focused by the focusing lens system. The object to be scanned is passed through the focused beam and if the bar code is sufficiently close to the beam focal point, reflected light from the bar code may be detected resulting in a successful scan. The scanning device may comprise moving spot laser beams from a laser or laser diode with the beam(s) scanned in one or more various directions out through the scanner window, CCD's, other imaging devices or a non-moving beam, such as a wand, in which the wand is moved across the bar code symbol or the object itself is moved past the beam in order to sweep the code symbol. As any optical system, data reading devices such as bar code scanners depend upon focused optics for effective and accurate performance.
As known by one skilled in the art, a focal point is typically not a discrete point but may be referred to as a "waist." The waist is the position along the beam axis where the "cone" of light from the light source reaches a minimum spot size, usually as measured in a direction parallel to the direction of spot motion. When the bar code or object being scanned does not fall sufficiently close to the focal point or waist, that is when the beam spot at that position is too large, the scanner cannot successfully read a symbol. By way of example, in a supermarket checkout application, a product bearing a UPC bar code label is passed at a certain distance in front of the window of a checkout scanner. The checkout scanner is designed with a scanning beam with a waist of a given diameter positioned at a certain distance from the window where the bar code is expected to pass. The checkout clerk generally becomes familiar with the proper distance to pass the object in front of the window, that is, the bar code must pass sufficiently close to the scanner focal point or waist (i.e. within its depth of field) in order to achieve a successful scan.
Though it is known that temperature can have an effect on focal properties of the scanner, the impact has been considered relatively small such that it has been deemed unnecessary to provide for significant temperature compensation. In a typical laser diode system, the diode produces a laser beam which is focused by a glass lens. Currently the lens barrel of the laser diode in the PSC Inc. model VS-1200.TM. bar code scanner is designed/optimized to offset the focal shift due to the thermal expansion of the glass lens.
The present inventor has determined that temperature may have a significant impact on the focusing properties of the reader and thus the focal distance when a plastic lens is used. Depending on the application, a scanner may experience significant temperature variation and thus produce a significant impact on focusing.